Lower the Polycount while keeping the details of sculpting
The purpose of multi resolution sculpt was so that artist could make a high res model. Then bake those information onto a low resolution mesh. "Blender Normals baking" or "Blender Baking Normals" will be the key word you want to search for online videos to learn about the technique. You will also need to understand how to prepare UVs for texture baking which is an essential step before you could produce good looking normals.

All of today's next gen games and games engine real time rendering utilise the normals map to give low res model and high resolution look.

Here is a nice concise read about normals, link below.

http://www.falloutsoftware.com/tutorials/gl/normal-map.html

Here is a nice simple tutorial video on baking a high multi res model's normals to be used on a low resolution model.

https://www.youtube.com/watch?v=xa6132QM2Y4